2002 Dodge Intrepid Engine won't rev up

I recently changed the water pump and pasenger side head gasket on my 2002 intrepid with a 2.7 liter engine. After every thing was put back together the engine starts up and idles fine but will rev up to just under 2500 rpm and die down, then back up to just under 2500 rpm and die down, and so on,
with the pedal to the floor. What could be the problem with the engine not reving up all the way ?